Embodiments of the inventive concept relate generally to semiconductor devices. More particularly, embodiments of the inventive concept relate to memory systems using a priori probability information to read stored data and methods of operating the memory systems.
Semiconductor memory devices play a significant role in a wide variety of consumer and industrial technologies, ranging from home computers to satellite equipment. Consequently, improvements in semiconductor memory technology can have a significant impact on the performance of numerous technical applications.
Semiconductor memory devices can be roughly divided into two categories based on whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Examples of volatile memory devices include dynamic random access memory (DRAM) and static random access memory (SRAM), and examples of nonvolatile memory devices include read only memory (ROM), ferroelectric random access memory (FeRAM), phase-change random access memory (PRAM), and flash memory.
Flash memory has become an especially popular form of nonvolatile memory due to its relatively low cost, low power consumption, high integration density, and ability to withstand physical shock. However, as the demand for flash memory continues to increase, researchers continue to seek ways to increase its storage capacity and performance.
One technique that has been developed to increase the storage capacity of flash memory is to store multiple bits of data in flash memory cells. A multi-bit flash memory cell stores multiple bits of data using multiple threshold voltage distributions that correspond to different values of multi-bit data. For instance, a 2-bit flash memory cell can store data using four different threshold voltage distributions each corresponding to one of logical states “11”, “10”, “00”, and “01”.
The performance and reliability of multi-bit flash memory cells tends to decrease as the number of threshold voltages increases. For instance, in a multi-bit flash memory cell that stores data using a large number of threshold voltage distributions, the threshold voltage distributions can become overlapped, which can change stored data. Accordingly, researchers continue to seek ways to improve the reliability of multi-bit flash memory cells.